Known fiber-optic current sensors operate on the principle of Faraday effect. The current flowing in an electric conductor (wire) induces a magnetic field, which through the Faraday effect rotates the plane of radiation polarization propagating in an optical fiber wound around the current-carrying wire. According to the theorem of magnetic field circulation, the following is obtained:Hdl=I  [1]
Herein I denotes the electrical current, H denotes the magnetic field and the integral is taken along a closed path around the current-carrying wire. If the sensitive fiber with constant sensitivity along the length to the magnetic field is wound around the wire, with the current as a circuit with an integral number of turns N, then the plane rotation of radiation polarization at the circuit's output depends on the current in the wire and is independent of all external magnetic fields generated. Such external magnetic fields include, for instance, currents generated by adjacent wires. A rotation angle of the polarization plane is equal to:φ=VHdl=VNI  [2]
Herein V denotes the Verdet constant for the fiber material, which may be silica, quartz, glass or polymer, for example. The sensitive optical fiber performs a linear integration of the magnetic field along its path; the integral is proportional to the current in the wire when this path is closed on itself. Rotation of the radiation polarization plane due to the presence of electric current is measured by the introduction of radiation with linear polarization in the sensing fiber loop, and subsequent analysis of the state of polarization after it gets out of the fiber loop. From a physical point of view, polarization plane rotation is caused by the fact that the two circularly polarized in opposite directions and equal in magnitude radiation components, the sum of which forms a linearly polarized radiation, have different propagation velocity in the sensing fiber located in a longitudinal magnetic field, which—after passing through the sensitive fibers—results in the appearance of (Faraday) phase shift between them, equal toφF=2φ  [3]
In current-sensing applications, wherein the state of circular polarization is indicative of the current being measured, the sensitive fiber should maintain the state of circular polarization.
Fiber-optic current sensors measure an angle of rotation φ, or equivalently, a phase shift φF. A well-known instrument for measuring current is a reciprocal reflective optical interferometer, an example of which is disclosed in reference document [1] (Laming et al.). This prior art instrument includes a measurement sensing fiber loop made of a sensitive optical fiber connected to a polarizer on the one end of the fiber and to a light reflector (mirror) on the other. Between the polarizer and the sensitive circuit, a beam splitter (directional coupler) is installed to branch the radiation to the device for analyzing rotation of light polarization plane after passing through the sensing fiber, first forward and then, after reflection from the mirror, backward. Sensitive optical fibers can be categorized into two types. The first type includes the optical fiber with very low linear birefringence (LB type), the second—magnetically sensitive optical fibers with embedded linear birefringence (spun fiber). The second type of fiber is obtained by drawing of a preform with a strong built-in (embedded) linear birefringence and a rotation of this preform during drawing process. In the present context, a preform means material that has undergone preliminary shaping but is not yet in its final form. As disclosed in reference document [1], the sensing loop of the current sensor employs a magneto-sensitive fiber with embedded linear birefringence.
Reference document [2] (U.S. Pat. No. 6,188,811) discloses a fiber-optic current sensor, which includes a light source, a polarizer coupled with the light source, a piezoelectric or electro-optical modulator of radiation polarization coupled with the polarizer, a fiber maintaining the linear polarization of radiation (“PM fiber”) and combined with the polarization modulator, a polarization-maintaining quarter-wave plate connected to the PM fiber, a magneto-sensitive fiber with embedded linear birefringence (spun fiber) combined with the quarter-wave plate, which to a large degree preserves the state of circular polarization, an output reflector coupled with the aforementioned magneto-sensitive fiber, and a photodetector with an output coupled with the polarizer. The magneto-sensitive fiber forms a sensing loop around the wire with flowing current.
FIG. 1A of said U.S. Pat. No. 6,188,811 shows an example of a linear current sensor. Light from a light source propagates through a coupler and a polarizer to a 45-degree splice, where it divides equally into the two polarization states maintained throughout the rest of the optical circuit. A piezoelectric birefringence modulator differentially modulates the phases of the light for the two polarization states. The piezoelectric birefringence modulator is driven by a modulator signal generator that provides an electrical, periodic, alternating signal having the shape of a square wave or sine wave. The light then propagates through a delay line and through a mode converter, which converts the two linear states of polarization into two circular states of polarization, and through an optimized sensor coil. The optimized sensor coil is wound around the current carrying wire. The light reflects off a reflective termination and retraces its way through the optical circuit, finally arriving at a photodetector. An open-loop signal processor converts the detected signal to an output, which is indicative of the current flowing in the current carrying wire. The sensor achieves its greatest sensitivity when the circular states of polarization are well maintained throughout the sensing coil. It is well known in the art that a spun birefringent fiber can preserve a circular state of polarization to some degree. For the invention disclosed in reference document [2], the concern is that the circular state of polarization be extraordinarily well maintained so that a very long length (hundreds of meters) of sensing fiber can be used. A straight spun birefringent fiber does hold a circular state of polarization over a long distance, but achieving this property is much more difficult when the fiber is bent, as is done when it is wrapped around a current carrying wire.
As stated in said U.S. Pat. No. 6,188,811, to obtain a high sensitivity of the sensor, the circular polarization state should be well maintained throughout the sensing circuit. It is well known in the art that a direct spun fiber is capable of maintaining the state of circular polarization over lengths of hundreds of meters. However, with a bend radius of less than about 20 mm, the sensitivity of the known fibers rapidly drops. For a given length of magneto-sensitive fibers, an increase in the allowable number of fiber turns in the circuit, and hence the sensor sensitivity, is limited. The fiber-optic sensors described in U.S. Pat. No. 6,188,811 overcome many of the shortcomings inherent in traditional single continuous fiber sensors. However, certain issues remain that affect the accuracy of the sensor. For example, to obtain very accurate measurements, optical components, in particular the quarter-wave plate should be ideal and not susceptible to external influences such as temperature alterations and mechanical disturbances. It is well established that the ideal or nearly ideal quarter-wave plates are difficult and very expensive to produce in order to achieve accurate reading required in certain applications. Known piezoelectric and electro-optical modulators have a residual level of parasitic modulation, which reduces the accuracy of the sensor. For effective modulation of known optical systems, a considerable length of PM fiber (up to 1 km) in the delay line is required. An associated problem is that PM fiber is expensive.
In addition, there is a need for fiber-optic current sensors of very high sensitivity. For instance, in some applications it is required to control small leakage currents in the lines with high rated currents, such as underground cables.
Finally, one of the problems in known fiber-optic current sensors is the dependence of magneto-optical sensitivity of the fiber loop on the external temperature and mechanical stress on the fiber. This dependence limits the accuracy of the sensor in the presence of these effects and requires a complex set of protective measures for sensitive circuits.